GPS Chart Plotter Different Position than Computer

[Jon D]

While on a two week cruise of Southern New England in August I noticed an interesting thing and curious if anyone else has observed same.

My Simrad Chartplotter is interfaced to my computer running a charting program [ The Captn]. At some points during the trip the GPS plotted us in different positions on the chartplotter than on the computer.

Obviously the GPS data was the same however the charts put us in slightly different spots on the ocean or ground in some cases. One of the more obvious examples is that as we were going through the C&D the Chart Plotter has a nicely following the edge of the canal the computer about 100 yards inland nicely following the contour.

My only thought is that the charts I was using on the computer might be older therefor not as accurate?

Any thought or comments.

[Vasco]

Jon,

According to my chartplotter I went over a couple of islands in the Bahamas. It's just bad charts. I was using Navionics which was better than C-Maps (my buddy was running that on similar (Raymarine) chartplotter) C-Maps had very little information on their Bahamas charts - a lot of blank areas. The best charts for the Bahamas anyway are the Explorer charts. The rotten thing about these chips is you don't really know what you're getting when you buy them. You can look at them in a demo mode at boat shows but until you're right there you can't tell how accurate they are.

[Greg B]

I experienced the same thing while cruising the banks of the TN river. Well I can clearly see that I am not on the bank but clearly in the channel. I run The Capt'n too but I don't blame that software. My guess was that the capt'n got the charts from a third party supplier whose calibrations were off. They used the same chart book that I had in hard copy and I scanned and uploaded some charts of the TN river that were not included with Capt'n. Those you have to calibrate yourself which I did and had no problems. Now it makes me wonder, can I recalibrate the charts that came with the software? Anyway, that's what I think.

GregB

[Talbot]

The most usual cause of a misalignment between plotters is an incorrect use of the Chat datum. Make sure both are set to the system being used (which may well be different).

The second most likely cause is a calibration problem either by yourself, or the supplier, where the chart alignment and scale are calibrated to your system - this is normally not a problem with Vector Charts.

The third possible cause is related to the origin of the charts, there are a number of islands that have not been surveyed since the introduction of GPS. The relationship between the islands will be accurate, but the overall position may be inaccurate either due to a sextant error, or possibly even a geodetic transfer error.

[Alan Wheeler]

Good comments Talbot. And to add a fourth. GPS can aslo be out. Accuracy of GPS can be out as much as 30-40m(100ft'ish). This can be for several reasons. Poor signal aqusition due to atmospheric conditions such as Rain, objects in line with antennea and satallite/s such as a sail and especially if sail is wet, satallite malfunction, not enough satallites being recieved. It takes a minimum of three for a position, but the more satallites recieved above three, the more accurate the position becomes. And of course the good ole military taking the whole set up "out of focus" so to say, for security reasons.
However Jon, for one chart to read different to another, I would be looking at your system setup. I don't know your programme, but you should have the ability to enter an exact Lat/Long correction figure somewhere, so as to align the chart to the GPS. Of course, you need to know of a physical place, the Long/lat for that point and you need to place your boat there.
And it may be as others have said, the fact that the charts are just incorrect. That's why they have warnings on the Echarts that they should not be relied upon as sole navagational aid.

[Jon D]

It is kind of funny but sad in some regards.

The systems are set up correctly - GPS data is consistent accross both, map datums are the same, etc. On some charts one is right on target on others off on others. By the same token the other can be off on some charts right on on others.

I beleive the real issue the age of the chart data.

Some of my PC based charts are older versions and I probably need to look at buying new chart CDs if I want to be absolutly safe. Might see if they'll cut me a deal at the boat show. Also might be worth while to upgrade the base GPS as it is older technology. Does not take advantage of DGPS or WAAS.

[Vasco]

When I "ran over" (according to the chartplotter) the islands the correct datum was set on the plotter and it's a differential gps which is extremely accurate. I can only conclude that the chart was wrong. This is not unusual. When I first sailed the Bahamas in 1990 some of the charts I was using were from the 1800's and these were the latest!! The new charts now are a lot more accurate but in the final analysis when piloting you have to rely on your eyes and proper seamanship.

[Vasco]

Jon D,

One of the problems in getting new CD's is you still can't tell what you're getting as far as accuracy goes. They may be corrected and updated with the latest notices to mariners but there is no guarantee that the charts are any more accurate than your old ones. It might be adviseable to talk to someone that has actually used the "new" charts especially in areas where you think there's errors before buying.

My Navionics chips of US East Coast and ICW are spot on (I've only used them from Charleston to Miami) but they are out in some parts of the southern Exumas. I now also have the Captn and some Explorer charts of the Bahamas and will be taking my laptop with me next trip. If I get around to it I'm going to interface it with a small Garmin as a backup. Of course I've got the Explorer paper charts, that's the real backup.

[Jon D]

Rick etc,

Following is a post I put up on another board --

"In reality most of the charts we use are less accurate than GPS is capable of.

My main reason for the post is that far "too many of us" tend to trust our electronics more than using visual confirmation, radar, depth, etc to insure that we are comfortable that we "know' where the boat is and what is a safe direction to procede.

I always use multiple inputs to validate where I am espcially if there are any 'hard' things to hit on the bottom like rocks, reefs, jettys etc..

It also goes to show that the latest technology does have it's cautions when using."

For me paper charts with visual reference is always the final check on any navigation. If something does not look or feel right I triple check myself until it does.

I have also found the newest charts from Softchart which combine vector and raster and run on the latest version of the Captn look really good and map to the explorer charts pretty well on first comparison.

[Talbot]

Any boat that navigates by GPS but doesnt keep an eye on at least the echosounder to correlate with your position by plotter, is not acting in a seamanlike manner (was going to write something else but decided that I had better not!!!!